1. Field of the Invention
The present invention relates to a process for separating radium from monocalcium phosphate solutions. More particularly, the present invention relates to a process for separating radium from monocalcium phosphate solutions by a centrifugation step.
2. Description of the Prior Art
Numerous prior art references describe many different wet processes for simultaneously producing both phosphoric acid solutions and calcium sulfate products (either the anhydrite form--CaSO.sub.4 with no water of hydration; the hemihydrate form--CaSO.sub.4.1/2H.sub.2 O; or the dihydrate form or gypsum--CaSO.sub.4.2H.sub.2 O). One generally disclosed type of process is to, first, digest phosphate rock with either recycled phosphoric acid, a recycled monocalcium phosphate (MCP) solution, or mixtures of both in order to form a monocalcium phosphate solution containing undigested gangue material (impurities such as Fe, Al, F and Si); second, separating the gangue from the MCP solution; third, reacting the clarified solution with sulfuric acid to simultaneously form a phosphoric acid solution and a solid calcium sulfate product; and, fourth, separating the phosphoric acid solution from the calcium sulfate product. As examples of this general type of process, see U.S. Pat. Nos. 2,531,977; 3,003,852; 3,418,077; 3,792,151; 3,840,639; 3,949,047 and 4,029,743. The milder rock digestion with H.sub.3 PO.sub.4 or MCP solution (rather than H.sub.2 SO.sub.4) allows for a relatively large portion of the undesirable impurities in the phosphate rock like Fe, Al, F and Si to be easily removed in the gangue.
However, the prior art references mentioned above were not generally concerned with the presence of radioactive impurities such as radium in the phosphate rock and usually did not teach any specific steps for their removal. Only U.S. Pat. No. 3,949,047, which issued to Cherdron et al. on Apr. 6, 1976, specifically teaches a method for removing radium contaminants from a monocalcium phosphate solution. That method includes the addition of a soluble barium compound in the presence of sulfate ions to precipitate radium ions. as radium sulfate. However, the use of barium compounds in large-scale commercial phosphoric acid plants seems prohibited by their relatively high cost.
The radium originating from the phosphate rock, if not removed, will usually end up in the calcium sulfate products (e.g., gypsum) made by the above-mentioned processes. In the past, these calcium sulfate products were generally discarded as unwanted by-products because of the presence of the impurities contained therein. Large piles of this material can be found at some phosphoric acid plants. Moreover, the presence of the radium in these calcium sulfate wastes has increasingly become the concern of governmental regulatory agencies and the industry itself. It is possible that these calcium sulfate wastes may become classified as hazardous materials if they have too high radium content. Furthermore, the radium imparts a slight radioactivity to these calcium sulfate products which poses a possible obstacle to their use in construction items such as wallboard, even when the radioactivity levels are minute.
Accordingly, there is a need in the art for an effective and economic means for removing at least a portion of the radium that originates in phosphate rock. Furthermore, there is a need in the art for making calcium sulfate products which have acceptable levels of radium so they may be utilized in construction items. The process of the present invention offers a solution to those needs.